Numerous cysteine proteases have been identified in biological systems. A "protease" is an enzyme which degrades proteins or peptides into smaller components. The term "cysteine protease" refers to proteases which are distinguished by the presence of a cysteine residue which plays a critical role in the catalytic process. Mammalian systems, including humans, normally degrade and process proteins via a variety of mechanisms including the actions of cysteine proteases. However, when present at elevated levels or when abnormally activated, or where introduced into a biological system in the context of a viral, bacterial or parasitic infection, cysteine proteases are thought to be involved in numerous pathophysiological processes and disease states.
For example, calcium-activated neutral proteases ("calpains") comprise a family of intracellular cysteine proteases which are ubiquitously expressed in mammalian tissues. Three major calpains have been identified: calpain I and II, and p94. The calpain family of cysteine proteases has been implicated in many diseases and disorders, including stroke, neurodegeneration, such as Alzheimer's disease, amyotrophy and motor neuron damage; acute central nervous system injury, muscular dystrophy, bone resorption, platelet aggregation, cataracts and inflammation. Calpain I has been implicated in excitatory amino-acid induced neurotoxicity disorders including ischemia, hypoglycemia and epilepsy. The cysteine protease p94, a muscle-specific member of the calpain family, has been identified as a gene product responsible for limb girdle muscular dystrophy (Barrett A. J., et al. ICOP Newsletter, 1-2 (1996)).
Lysosomal cysteine proteases or cathepsins (including cathepsins B, C, H, L, S, O and O2/K) belong to the papain superfamily of cysteine proteases. They are widely distributed and differentially expressed among tissues. Intracellularly, they serve a variety of digestive and processing functions. Extracellularly, they may be involved in tissue remodeling and in pathologies such as arthritis, inflammation, myocardial infarction, Alzheimer's disease, cancer and muscular dystrophy (Elliott E., et al., Per. in Drug Disc. and Des., 6:12-32 (1996)).
Interleukin-1.beta. converting enzyme ("ICF") is a member of the caspase family of cysteine proteases which catalyzes the formation of interleukin-1.beta. (IL-1.beta.), as well as the formation of interferon-.gamma. inducing factor (IGIF) from their inactive precursors, proIL-1.beta. and pro-IGIF, respectively. Interleukin-1.beta. is an immunoregulatory protein implicated in inflammation, diabetes, septic shock, rheumatoid arthritis and Alzheimer's disease. ICE and/or other caspases have also been linked to the apoptotic cell death of neurons which is implicated in a variety of neurodegenerative disorders including Parkinson's disease, ischemia and amyotrophic lateral sclerosis (ALS)(Dinarello C., et al., New Eng. J. Med., 328: 106-113 (1993)).
Cysteine proteases are also produced by various viral pathogens and appear to be involved in every stage of reproduction including DNA and RNA translation and synthesis, and capsid formation (Gorbalenya A., et al., Per. In Drug Disc., 6:64-86 (1996); Krausslich et al., Ann. Rev. Biochem., 57:701-54 (1988)). Examples of viral pathogens include Picornaviridae, which includes the genera Enterovirus, Rhinovirus, Cardiovirus, and Aphthovirus, which cause numerous human disease syndromes, ranging from fatal paralysis, encephalitis, meningitis, hepatitis and myocarditis to the common cold (Krausslich et al., Ann. Rev. Biochem., 57:701-54 (1988)). The picornaviral 3C proteinases, which are produced by all picomaviruses, are responsible for processing viral polyproteins, an essential stage in viral growth (Malcolm B., et al. Biochemistry, 34:8172-8179 (1995)).
In addition, parasitic cysteine proteinases play significant roles in host-parasite interactions and pathogenesis (Robertson C., et al., Pers. in Drug Disc. and Des., 6:99-118 (1996)). For example, most of the proteinase activity detected in trypanosomes and various Leishmania species has been characterized as belonging to the cysteine protease class. Other proteases are produced by Clostridium histolyticum and malaria parasites, such as Plasmodium falciparum and Plasmodium vinckei strains, and Schistosoma.
Cancer procoagulant, CP, a cysteine proteinase from malignant cells, has emerged as a probable activator of the coagulation system in cancer (Alessio M. G., et al., Eur. J Haematol, 45: 78-81 (1990); Gordon S., Methods in Enz., 244:568-581 (1994); Gordon S., Sem. in Thromb. and Hemo., 18,4:424-433 (1992)).
Existing cysteine protease inhibitors are primarily irreversible in nature; only weakly inhibit the enzymatic activity of the targeted protease and/or are toxic. Thus, there is a need for effective inhibitors of cysteine proteases as therapeutic and as prophylactic agents for the treatment and/or prevention of cysteine protease mediated pathologies.